Practical realization of high performance light energy conversion system utilizing renewable energy such as solar energy rapidly increases the importance in recent years from the standpoints of inhibition of global warming and the aim of departure from dependency of fossil resources that are running out. Above all, the technology of splitting water using solar energy to produce hydrogen is not only a technology of the existing petroleum refining and raw material supply of ammonia and methanol, but a technology required in the upcoming hydrogen energy society based on a fuel cell.
Water-splitting reaction by photocatalyst is widely studied from the 1970s (Non-Patent Document 1). Many photocatalysts had the disadvantages that due to large band gap, water splitting proceeds if in ultraviolet region, but visible light region cannot be utilized, and even through the visible light region can be utilized, the catalyst itself is unstable in water. However, on or after 2000, water can be split by light energy in a visible light region, and photocatalyst stable in water, that is, visible light photocatalyst, began to get published
For example, oxynitride, nitride, oxysulfide and sulfide are known as the visible light photocatalyst (Non-Patent Document 1).
Water splitting method by photocatalyst is greatly classified into two kinds. One is a method of conducting water splitting reaction in a suspension, and another is a method of conducting water splitting using an electrode comprising a conductive metal substrate and photocatalyst deposited on the substrate in a thin film form, and a counter electrode.
The former is that photocatalyst capable of performing complete splitting of water is currently limited, and additionally, hydrogen and oxygen that are products of water splitting reaction are formed as a mixed gas, and this requires separation of hydrogen and oxygen after recovering. On the other hand, the latter has the advantage of having many selections of photocatalysts that can be used. For this reason, in recent years, an example of forming visible light water splitting photocatalyst in a thin film form and carrying out water splitting in high efficiency is reported (Non-Patent Document 2).
Generally, deposition of photocatalyst on a conductive metal support is conducted by physical vapor deposition (PVD) methods represented by a vacuum deposition method and a sputtering method and chemical vapor deposition (CVD) methods (those methods are generically named dry process); coating methods represented by spin coating and screen printing; sol-gel methods; and electrophoretic deposition (EPD) methods (Non-Patent Document 3 and Patent Document 1).
On the other hand, a method of covering the circumference of photocatalyst particles with semiconductor or good conductor, having conductivity higher than that of photocatalyst used, in order to improve electron conductivity between photocatalyst particles and between photocatalyst and a support is proposed. For example, a method of covering the circumference of iron oxide or tungsten oxide particles that are photocatalyst with oxide semiconductor of titanium, aluminum, antimony, tin, zinc, zirconium or the like to improve conductivity is proposed (Patent Document 2).
However, where oxides of metal species different from photocatalyst, such as titanium oxide are used as a conductor to iron oxide or tungsten oxide that is photocatalyst, the respective energy levels greatly differ. This gave rise to the problem that mismatch occurs in level, causing resistance, and conductivity is decreased.
To solve the above problem, in an electrode for water-splitting reaction comprising a support having deposited thereon at least one kind of photocatalyst particles selected from the group consisting of oxynitride, nitride, oxysulfide and sulfide, an electrode for water-splitting reaction having semiconductor or good conductor between the photocatalyst particles and between the photocatalyst particles and the support is disclosed (Patent Document 3). According to this disclosure, semiconductor or good conductor is present around the photocatalyst particles, and as a result, internal resistance of the electrode is decreased, and photoelectric conversion efficiency can be improved.